To safely request and drop privileges, runtime.Lock/UnlockOSThread and
windows.Impersonate/RevertToSelf should be called. Add these calls to
winutil.EnableCurrentThreadPrivilege so that callers don't need to worry
about it.
Updates https://github.com/tailscale/corp/issues/15488
Signed-off-by: Andrew Lytvynov <awly@tailscale.com>